FFMPEG from Inside Blender
Here's a solution to launch ffmpeg automatically following Blender's completion of rendering out an image sequence:

import bpy
import os

bpy.ops.render.render(animation=True)

ffmpegPath = "YOUR-PATH-TO-FFMPEG-APP"

imgPath = "YOUR-PATH-TO-IMAGE-OUTPUT-FOLDER"

#frame filename format assuming "0001.png"
imgNamePattern = "%04d.png"
vidOut = "test.avi"
vCodec = "mpeg4"

ffmCMD = ffmpegPath 
ffmCMD += " -start_number 1 -i " 
ffmCMD += imgPath + imgNamePattern 
ffmCMD += " -vcodec " + vCodec + " " 
ffmCMD += imgPath + vidOut

print("launching ffmpeg")
os.system(ffmCMD)